1. Field of Invention
The present invention relates to a mount for a flip-chip architecture semiconductor light emitting devices such as light emitting diodes.
2. Description of Related Art
Light emitting diodes (“LEDs”) are solid-state light sources with multiple advantages. They are capable of providing light with high brightness reliably and thus find applications in displays, traffic lights, and indicators, among others. An important class of light emitting diodes is fabricated from one or more Group III elements, such as Gallium, Indium, or Aluminum, and the group V element of Nitrogen. These III-nitride LEDs are capable of emitting light across the visible spectrum and into the ultraviolet regime of the spectrum, and thus have many promising applications. Other light emitting diodes may be made from 11-phosphide and III-arsenide materials systems, which emit in the amber, red, and infrared regions of the spectrum.
Traditionally, LEDs are fabricated by depositing an n-doped region, an active region and a p-doped region on a substrate. Some LEDs have an n-contact formed on one side of the device and the p-contact is formed on the opposite side of the device, creating a vertical device. Other LEDs have both contacts formed on the same side of the device, with light extracted through the contacts. Such a structure is referred to as an epitaxy-up device. In both a vertical device and an epitaxy-up device, much of the light generated by the active region exits the device through the p-contact. Since the p-contact typically includes a metal and/or a semi-transparent metal oxide in order to optimize its electrical conduction properties, the p-contact generally transmits light poorly, posing a design problem.
Recently, a flip chip architecture has been proposed in relation to this design problem. As shown in FIG. 1, in a flip chip device 100 the die 110 is mounted on a submount 114 with the contacts facing toward the submount. Much of the light generated by the active region exits the device through the substrate, rather than the contacts. All of the electrical contacts can be positioned on the bottom of flip chip dice. The device is completed by forming the submount 114, solderable layers 118-1 and 118-2 overlying the submount, and solder balls 122-1 and 122-2 on the solderable layers, and then attaching the die 110 to the solder balls 122 to provide electrical contact for the die.
Existing designs provide a path for the current by placing wire bonds in electrical contact with the solderable layers. The wire-bonds consist of balls 126-1 and 126-2 formed on the solderable layer, and connected wires 130-1 and 130-2. The wires are then connectable to the package leads 132-1 and 132-2 of the package of the light emitting device. The submount and the die itself are attached by a die epoxy to the wiring board 134.
Wire-bonded devices have several drawbacks. First, the wire bonds are sensitive to heat. One of the limitations of the LED design is thus how much heating the wire-bonds can endure. This issue becomes more and more important as newer generations of LEDs are planned to be operated at higher power and in higher temperature environments, leading to an increase in operating temperatures and heat production. The currents in the wires heat up the wires, a process referred to as ohmic heating. The degree of the ohmic heating is determined, among other things, by the current density. Elevated temperatures and repeated thermal cycling can lead to damage to the wire bond, such as separation of the ball from the solderable layer, brittleness in the wire, or breakage in the wire caused by melting at a narrow cross section. Such heating problems can also occur in case of an electrostatic discharge (“ESD”), or during transient periods, such as switching the device on and off. Elevated temperature operation can also lead to enhanced growth of physically brittle and electrically resistive intermetallic phases at the interface between balls 126 and solderable layers 118, which can ultimately cause failure at the interface.
Second, the wire-bonds do not conduct a significant amount of heat away from the LED. Heat is generated within the LED during regular operation. Light is generated in the LED by electrons from the n-layer recombining with holes from the p-layer. Some of this recombination is radiative, leading to the emission of photons. However, a sizeable fraction of the recombination may be non-radiative, generating heat instead. In some devices, at least some of the heat generated within the device must then conducted away from the die to avoid damaging the LED. The wire-bonds have a relatively small cross-sectional area and thus do not contribute significantly to conducting heat away from the die.
Third, the wires are fragile and thus are usually the primary failure mechanism under extreme operating conditions, such as temperature shocks, rough handling, and high humidity environments. In order to protect the fragile wire-bond, the LED must be assembled in a package to be of practical use for the end users.
Fourth, wire-bonds require space on the submount outside the footprint of the LED, resulting in bigger and more expensive devices. Light emitted by the LED and incident on the large submount may be reflected from the submount, undesirably increasing the size of the light source.
Finally, wire-bonds can trap air in the package during subsequent encapsulation of the LED, creating large steps in index of refraction which may reduce the efficiency with which light is extracted from the LED die into the outside world.